The most current version of OS X is OS X 10.9 Mavericks. OS X Mavericks is available as a free download from the Mac App Store. If you need to purchase Mac OS X 10.6 Snow Leopard you may order it from this page.
Installing Snow Leopard: What you need to know
With Snow Leopard, Apple has made the process of upgrading OS X easier and more streamlined than ever. Dan Frakes explains what you need to know and how to make the upgrade work for you.